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Fair value measurements - Other information (Detail) $ in Millions | 9 Months Ended |
--- | --- | ---
Sep. 30, 2025 USD ($) | Dec. 31, 2024 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments in equity securities | $ 283,241 | $ 271,588
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments in equity securities | 9,806 | $ 9,668
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] | Discounted Cash Flow [Member] |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments in equity securities | 8,780 |
Significant Unobservable Inputs (Level 3) [Member] | Common Stock Warrants [Member] | Fair Value, Measurements, Recurring [Member] | Warrant Pricing Model [Member] |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments in equity securities | $ 1,019 |
Significant Unobservable Inputs (Level 3) [Member] | Measurement Input, Expected Term | Weighted Average [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] | Discounted Cash Flow [Member] |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Unobservable inputs, term | 4 years |
Significant Unobservable Inputs (Level 3) [Member] | Measurement Input, Expected Term | Weighted Average [Member] | Common Stock Warrants [Member] | Fair Value, Measurements, Recurring [Member] | Warrant Pricing Model [Member] |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Unobservable inputs, term | 5 years |
Significant Unobservable Inputs (Level 3) [Member] | Discounts for Liquidity and Subordination [Member] | Weighted Average [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] | Discounted Cash Flow [Member] |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Unobservable inputs | 0.0325 |
Significant Unobservable Inputs (Level 3) [Member] | Measurement Input, Option Volatility | Weighted Average [Member] | Common Stock Warrants [Member] | Fair Value, Measurements, Recurring [Member] | Warrant Pricing Model [Member] |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Unobservable inputs | 0.43 |

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